1. Field of the Invention
The present invention relates generally to non-volatile semiconductor memory devices and, more particularly, to the use of tracking cells for the reading of multi-state memories.
2. Background Information
In a semiconductor memory cell, data is stored by programming the cell to have a desired threshold voltage. The data stored in the cell is read by determining the threshold voltage of the cell and translating this voltage to a logic level. For a two state, binary memory cell, this translation can be done by use of a reference or breakpoint voltage to provide a read point: cells with a threshold voltage above this read point correspond to one state, while those with a threshold voltage below this read point correspond to the other state. When a memory cell is a multi-state cell, a number of such read points need to be introduced to distinguish between the states.
As the number of states stored in a memory cell increases, more states must be contained within a range of threshold values. Consequently, the portion of this range corresponding to a given state become ever narrower and the read points become ever closer. Once the memory cells have been programmed, their threshold values may change from the level to which they were programmed for a number of reasons. This can be due to the less than perfect charge retention in non-volatile memories, where a cell's threshold may go up or down depending on applied bias voltages, fields generated by the stored charge itself and charge trapped in dielectrics. It can also be due to changes in operating conditions between when the cell is programmed and when it is read. If the read points used to determine the data content of the cells do not follow these changes in the cell, the read points will no longer accurately discriminate between the different threshold voltages. This is the case when a fixed, global set of reference voltages, such as from a band-gap circuit, is used to produce the read points. Such a circuit will not respond, or respond differently, to the conditions which lead the threshold voltages of the memory cells to change. When this change becomes large enough, the read points will no longer accurately discriminate between the threshold voltages found on the cells and the data programmed can no longer be accurately read.
One method to improve the accuracy of the correspondence between the read points and threshold voltage of the cells programmed to a particular data state at the time these cells are read is the use of writable reference or tracking cells. These are a set of memory cells, but which are not written with data and instead are written to predetermined reference values. The read points are then extracted from these cells, which, as they will behave similarly to the data cells, will provide a more accurate correspondence between a data level and the current threshold voltage of a cell originally programmed to that level. The use of reference cells in multi-state memories are described in U.S. Pat. No. 5,172,338 and further developed in U.S. patent application Ser. No. 08/910,947, filed on Aug. 7, 1997, both of which are assigned to SanDisk Corporation and both of which are hereby incorporated herein by this reference.
As the number of states per cell continues to increase, further improvements in the accuracy of the reading process will be needed. Therefore, improvements in tracking cell techniques are needed which not only increase their accuracy and speed of use, but also decrease the amount of overhead they require, both in terms of the actual tracking cells and also in terms of the related circuitry.